A well-known use of phosphors is in the production of X-ray images. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted image-wise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
According to another method of recording and reproducing an X-ray pattern as disclosed e.g. in U.S. Pat. No. 3,859,527 a special type of phosphor is used, known as a photostimulable phosphor, which being incorporated in a panel, is exposed to incident pattern-wise modulated X-ray beam and as a result thereof temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which can be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
In both kinds of radiography it is preferred to be able to choose the phosphor that will be used on the basis of its speed and image quality without having to bother about its hygroscopicity.
Therefore it is highly desired to have the possibility of producing a phosphor panel, be it for use in direct radiography or in computed radiography, that is impervious to water vapour. In U.S. Pat. No. 4,741,993 a radiation image storage panel is disclosed having at least one stimulable phosphor layer on a support and a protective layer provided on the stimulable phosphor layer, wherein the protective layer comprises at least two layers of which regains under a relative humidity of 90% on a sorption isotherm at 25° C. are different by 0.5% or more. According to that invention, a radiation image storage panel which has good humidity resistance and can be used for a long term is obtained. Although a protective layer as disclosed in U.S. Pat. No. 4,741,993 does provide good humidity protection, the need for providing phosphor panels with even better humidity resistance is still present.
Use of parylene layers as humidity protective layers of hygroscopic photo-stimulable phosphor screen layers has been disclosed in e.g. EP-A-1 286 362 and EP-A-1 286 365. Also in DE-A-196 25 912 and GB-A-2 287 864 phosphor screens containing a parylene layer are disclosed.
Although screens prepared according to the disclosures above do yield screens with an acceptable to even a very good overall quality, the need for a phosphor screen combining good humidity resistance and good resistance against physical damage, especially scratch resistance during use is still present. This problem is posed the more after frequent use of desired CsBr-based phosphor plates and as a result thereof the image quality thereof is reduced as a function of time. When the plates have to be cleaned or when they are taken out of the cassette the parylene layer, normally offering an adequate protection against moisture for the hygroscopic CsBr-based phosphor, may be damaged because parylene has a very low scratch resistance resulting again in a stability problem of the image plate.
Coating of a layer onto the parylene layer is thus highly desired as has e.g. been described in EP-A-1 286 364. From that radiation cured polymeric layer coated onto said parylene layer in order to further essentially prevent the phosphor plate against physical damage as a scratch resistant layer it is known that, over a long period of frequent use, adhesion of organic or inorganic polymer layers towards parylene coating is low. As a consequence adhesion problems may occur in the course of the life time of an imaging plate, especially after frequent use.